"Gudule" is not really a demo in the common sense of the word. It doesn't have any effects, a 3D engine, or the usual stuff one finds in demos. Instead it features many small animations of different characters. All of these animations features small cartoon characters.
You can now wonder what makes "Gudule" interesting. Well, it can not be the effects, as there aren't any. It is simply the strange style and the humour of the production. When you watch it the first time, you will most probably either hate it or laugh at it. As for myself, I found it a bit funny the first time I watched it, as it was so incredibly different from what one normally see in demos.
Also, in the background there is a French guy singing, so it is probably funny for those who are familiar with that language. For the rest of us, I guess we just have to guess what he says.
All in all, "Gudule" is a different kind of demo. Not anything spectacular, but still worth seeing because of the cartoons and the snail at the beginning of the demo.